1. Field
The invention relates to a video camera pickup tube circuit which automatically selects the mode of operation of the tube in response to the presence or absence of excessive highlights in the viewed scene.
2. Prior Art
The level of limitation of the dynamic range of a Plumbicon pickup tube is determined by the maximum beam current that can be applied without causing unacceptable discharge lag, loss of resolution, or loss of registration. Highlights in excess of the beam current setting cause local unstabilization of the Plumbicon target. Intensely illuminated target elements lose more charge between successive scans than the limited beam can replenish and therefore their potential rises towards that of the signal plate. As the scanning beam approaches or leaves the resulting high potential highlight area it is attracted or retarded respectively by the greater potential. Thus, the beam is pulled ahead of or behind its normal time-position track.
When a highlight is fixed in position with relation to the camera, particularly when viewing intense highlights, the corresponding target elements and also the adjacent elements become almost fully discharged. With the added possibility of optical dispersion within the layer itself, the cumulative effect is to enlarge the unstabilized target area around the highlight, thus causing the associated picture area to appear vacant or washed-out.
A moving highlight, caused by movement in the scene or by panning the camera, requires many scans before the successively discharged elements are stabilized. Each scan produces a highlight diffusion in a fractionally different position thus causing a blur in the picture known as a comet-tail. The length, and intensity, of the comet-tail depends on factors such as highlight intensity and velocity, target storage capacitance and voltage, and beam current.
Recently it has been found that the disadvantages of excessive highlights may be allayed by the introduction of a high intensity beam during the line flyback period of the pickup tube. The intensity of this beam is increased to allow it to replenish the majority of discharges caused by highlights to effectively limit the potential excursions of associated target picture elements to an acceptable peak signal level. These are then stabilized by the subsequent normal read-out scanning beam. However, the use of the high intensity beam to replenish the discharges is disadvantageous in that it also generates an excess of soft X-rays which bombard the target, thereby significantly reducing the lifetime of the pickup tube. To prolong its lifetime, the tube thus must operate with its target at a decreased potential of the order of 750 volts. This compromise in turn causes an undesirable decrease in resolution and increased sensitivity of the scan beam to deflection by the earth's magnetic field, while still causing a reduction in the lifetime of the tube due to the soft X-rays generated by its generally continuous operation.
A tube employing the high intensity beam is controlled by a circuit which provides suitably-timed pulses to the various electrodes of the tube to generate the high intensity beam during the line flyback period. However, the high intensity beam is generated during every line flyback period of every field even in the absence of excessive highlights. This continuous operation causes the aforementioned significant reduction in the tube lifetime since soft X-rays are generated during every flyback period, even though the tube mesh is operated at the (less desirable) reduced potential.